A fibre-reinforced composite works because thin fibres of glass or plant cellulose are far stronger than the same material in bulk, and the resin around them transfers load into each fibre through shear at the surface. Strength is highest along the fibres and depends on how much fibre is packed in, how well the resin wets it, and how few air voids remain. A reinforcing fibre is not a rope fibre: it must be clean, aligned and bonded, not twisted and oiled for spinning.
Part of Composite Materials
A fibre-reinforced composite is a resin matrix, such as polyester, epoxy or a natural binder, containing strong fibres that carry most of the tensile load.
Fibres Are Strong Because They Are Thin
A fine glass fibre is much stronger than a rod of the same glass because a thin fibre has few and tiny surface flaws (see Advanced Ceramics for why flaws set strength). The Advanced Materials article gives glass fibre diameters of 10-20 micrometres. In the composite, the resin does what a bundle of loose fibres cannot: it keeps each fibre separate and straight, shares load among them when one breaks, and protects them from abrasion and moisture.
Load Gets Into The Fibre Through The Interface
Load enters a fibre by shear along its surface. A fibre must be long enough that the bond can load it to its full strength, so continuous or long fibres and cloth give far more than short chopped strands. If the bond is poor or the resin has not wet the fibre, the fibre pulls out under load and the composite fails at a fraction of its potential.
The Variables That You Control
- Fibre fraction. More fibre, more strength, until there is not enough resin to wet it. The Advanced Materials article’s glass-resin target is covered in Fiberglass.
- Orientation. Cloth with fibres in two directions carries load both ways. Mat is random and equal in all directions but weaker.
- Voids. Air bubbles are flaws. Roll them out as the Advanced Materials article instructs.
- Cure. A resin that has not fully cured is weak and may stay tacky.
Natural And Glass Fibres Are Different Tools
The Advanced Materials article presents two routes: Fiberglass, for the highest strength and water resistance, and Natural Fiber Composites, using flax, hemp and jute where glass is unavailable. Note the trap on names. The corpus articles named fiber sources and plant fibres are about papermaking and rope respectively, and neither is a composite reinforcement. Rope fibre is twisted and often treated for spinning, which works against a clean bond and straight loading.
Worked Example: Fibre Needed For A Panel
A flat panel 500 mm × 300 mm × 3 mm has volume 500 × 300 × 3 = 450,000 mm³ (450 cm³). At 30% fibre by volume that is 135 cm³ of fibre. With glass at roughly 2.5 g/cm³ (typical value, not a corpus number), that is about 340 g of glass cloth, plus the resin to fill the remaining 315 cm³. Weigh the cloth before laying it to know what you have used.
Resin fumes, fibre dust and exotherm
Uncured resins and their vapour irritate the lungs, eyes and skin, and glass and plant dust from cutting and sanding irritate the skin and lungs. Work outdoors or with ventilation, wear gloves, long sleeves and eye protection, and a mask when sanding. Thick resin masses heat as they cure and can smoke or ignite, so mix small batches and spread them thin.
Common Mistakes
- Dry fibre on top — it is unprotected, and the laminate delaminates.
- Cutting fibres across the load path — it loses the strength you built.
- Using any fibre lying around — a reinforcement must be clean and straight.
What’s Next
- Learn next: Fiberglass — the practical glass-and-resin composite
- Combine with: Epoxy Systems — a stronger matrix